What is the context here? Are they adding the eggs to something, or aare they doing arithmetic with eggs? I answered "They are increasing the eggs," which of course makes no sense, but it accepted it. What sense of "adding" does "-ongeza" convey?
Why can't the answer be "They are adding eggs"? Why is "the" required here?
It's just because not all options have been added to the course yet. Either way is fine. If you see it again, make a report.